[fix](cloud) Fail closed on unknown MetaService response codes

[wyxxxcat]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary: Meta Service responses contain both an exact actual_code and a
legacy-compatible code. When a client recognizes the actual_code, it should use
that exact value. When actual_code is unknown, the legacy code is safe only if
it is explicitly present and non-OK. The previous logic could fall back to an
unset or OK code and incorrectly treat an unknown Meta Service error as success.
This change makes BE and FE fail closed with UNDEFINED_ERR in that case, while
preserving compatible non-OK fallbacks. It also adds a message explaining the
MS_TOO_BUSY to KV_TXN_CONFLICT conversion for legacy clients.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[wyxxxcat]

run buildall

[hello-stephen]

BE UT Coverage Report

Increment line coverage 100.00% (12/12) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	59.29% (25709/43361)
Line Coverage	43.34% (258037/595362)
Region Coverage	39.03% (204567/524141)
Branch Coverage	40.37% (93359/231234)

[hello-stephen]

FE UT Coverage Report

Increment line coverage 81.82% (9/11) 🎉
Increment coverage report
Complete coverage report

[wyxxxcat]

run buildall

[wyxxxcat]

run buildall

[hello-stephen]

Cloud UT Coverage Report

Increment line coverage 100.00% (8/8) 🎉

Increment coverage report
Complete coverage report

Category	Coverage
Function Coverage	77.57% (1912/2465)
Line Coverage	64.53% (34220/53027)
Region Coverage	64.44% (17278/26812)
Branch Coverage	53.94% (9251/17150)

[hello-stephen]

FE UT Coverage Report

Increment line coverage 81.82% (9/11) 🎉
Increment coverage report
Complete coverage report